Over 299,000 total knee replacement procedures were conducted in 2002 with projections to 474,000 by 2030. It is estimated that the annual cost of TKR exceeds $6 billion in the US. Despite the success of TKR revision surgery accounts for between 8 and 9 percent of the annual TKR procedures with 25,000 knee revisions conducted in the US in 1999 at a cost of $600 million. Clinical studies reveal that the need for revision is most often caused by polyethylene wear or other polyethylene damage modes. Clinical evidence also indicates that as implant technology evolves new damage and wear modes arise. The high incidence of TKR, the frequency, cost and severity of revision, and the evolution of implant designs together mandate rigorous implant life cycle testing in simulators capable of replicating the subtleties of human motion. Biomechanical arguments indicate that anterior-posterior force control and tibial rotation under torque control are the most realistic approaches for knee motion simulation. However implant wear testing is usually performed in the absence of the knee's supporting soft tissue which renders force and torque control impractical. In this proposal a real time impedance based feedback control system which incorporates a software embodied soft tissue model will be developed. The model will be based on a knee motion space in which a stiffness surface is defined. In Phase I the model will include nonlinear asymmetric ligament characteristics, coupled compliance flexion arc behavior and coupled flexion arc neutral position behavior. The model will be incorporated into a nested feedback control loop design suitable for control of simulator machine servohydraulic actuators. This improvement in control technology will benefit the affected population by providing a realistic environment in which to carry out implant life cycle testing with the ultimate objective of increased longevity of knee implant appliances. The phase I effort will establish proof of principle while providing the basis for extension in phase II to include rotational coupling and the more complex active musclo-tendinous structure. A successful phase I alone will improve the state of the art of knee implant testing.